docs: def set_add(self, key, value, create=False, **kwargs):
          """
          Add an item to a set if the item does not yet exist.
  
          :param key: The document ID
          :param value: Value to add
          :param create: Create the set if it does not exist
          :param kwargs: Arguments to :meth:`mutate_in`
          :return: A :class:`~.OperationResult` if the item was added,
          :raise: :cb_exc:`NotFoundError` if the document does not exist
              and `create` was not specified.
  
          .. seealso:: :meth:`map_add`
          """
          op = SD.array_addunique('', value)
          try:
              sdres = self.mutate_in(key, op, **kwargs)
              return self._wrap_dsop(sdres)
          except E.SubdocPathExistsError:
              pass